We use an optical fiber sensor whose part of clad is removed for the optical constants of ultra thin film measurement. The sensitivity of the sensor is increased by decreasing the diameter of the optical fiber and/or increasing the length of the film deposited part. The result of the in situ measurement method of change of amorphous hydrogenated silicon (a-Si:H) ultra thin films by oxidation with passage of time by leaking from vacuum to atmospheric air is shown. We use an optical fiber sensor whose length of the film deposited part is 100 mm, diameter of the core is 110 micrometers and the core material is silica. a-Si:H film with approximately 10 nm in thickness is deposited on the core of the optical fiber sensor. The oxidation saturates in approximately 600 s. The larger attenuation of transmitted light at the long wavelengths from 685 to 800 nm is observed. This phenomenon is maybe due to the change of defects by relaxation.